Efforts to reduce sulfur emissions in the gaseous products from combustion of sulfur containing fuels have been made for some time in an effort to comply with evermore stringent governmental regulations. Various processes have attempted to reduce or eliminate the sulfur in fuel prior to combustion, yet other processes propose the addition of compounds to the combustion zone which will in some manner change the nature of the sulfur compounds emitted such that such sulfur compounds may be more readily removed from the combustion process. Still other processes remove sulfur compounds from the gaseous combustion products by chemical reaction.
The approach of chemical treatment to effect flue gas desulfurization can be further subdivided into wet scrubbing wherein a wet solution or suspension of reagent enters and leaves the flue gas contacting zone in a liquid state or spray drying wherein a solution or suspension of reagent enters the flue gas contacting zone in liquid state but is subsequently dried to produce a powdered solid leaving the contacting zone.
Finally, dry treatments are utilized wherein the treating reagent is a solid state powder both entering and leaving the contact zone.
Representative but nonexhaustive examples of art in this area include U.S. Pat. No. 4,185,080 which discloses a combustion gas desulfurization process using a powdered sorbent such as limestone or dolomite added to a combustion zone wherein a portion of the solids collected from the flue gas is reactivated and then returned for injection into or downstream of the combustion zone.
Wet scrubber approaches can be illustrated by U.S. Pat. No. 3,928,537 which discloses contacting the exhaust gas with an aqueous solution of an organic acid to form a soluble sulfite or sulfate. The sulfite or sulfate is removed and the organic acid regenerated in a step comprising reaction with a calcium compound such as an inorganic or an organic acid salt as illustrated by calcium hydroxide or calcium formate.
Spray dryer methods are illustrated by U.S. Pat. No. 4,279,873 which discloses spraying a suspension of fresh, slaked lime and recycled flyash plus spent calcium compound into the hot flue gas in such a manner as to evaporate the slurry droplets to dryness. The resulting powdered solids are removed from flue gas by a downstream electrostatic precipitator or bag filter.
Dry treatment is illustrated by U.S. Pat. No. 4,178,349 which discloses mixing a dry powdered lime bearing material in a reactor and subsequently separating the solids from the treated flue gas. Further, U.S. Pat. No. 4,442,079 describes a flue gas desulfurization process which is primarily adapted to a spray dryer procedure. It is also taught to be applicable to injection of dry sorbent at a point of relatively low flue gas temperature with water sprayed into the gas either upstream or downstream of the sorbent injection point.
However, practical experience has taught that no single strategy as discussed above is the unique answer to sulfur emission problems, whether for new installations or for retrofit on existing facilities. In order to meet sulfur emission standards, numerous site specific factors such as proximity to reagent source, space availability and extent of sulfur removal required must enter into the selection at each combustion site.